This invention relates to a non-synthetic biodegradable starch-based composition and to the process by which this composition is transformed into various foam-liked products.
Environmental concerns associated with the handling of packaging waste have emphasized the importance of developing biodegradable materials to alleviate the waste disposal problems. Different approaches have been proposed. Blending of plastics widely used in packaging such as polyethylene, polypropylene, and polystyrene with biodegradable fillers, particularly starch, to make those plastics which are synthetic materials become biodegradable has not been overly successful. Synthetic biodegradable polymer such as polylactic acid, polycaprolactone as well as naturally occuring polymers i.e. polyhydroxybutyrate and polyhydroxyvalerate have been investigated to replace plastic in packaging areas. It has been found that these materials are generally deficient in properties required in packaging applications and excessively expensive.
Starch is of interest as a biodegradable material because of its availability as agricultural surplus raw material, its low cost and its thermoprocessibility. There has been much interest recently in developing starch-based package for single use, particularly foam-liked articles such as tray, cup, bowl and fast food box. According to its thermoplastic properties, starch can be expanded when it is heated above its glass transition temperature, in the presence of water and pressure and simultaneously formed into a desired shape.
Starch is a non-synthetic, bio-polymer, consisting principally of amylose and amylopectin. When starch is appropriately conditioned or formulated, it may be formed into foam-liked materials. The appropriate conditioning or formulating may relate to moisture, temperature, pressure, acid-base value and addition of some chemicals.
In Patent Cooperation Treaty (PCT) Publication No. WO 83/02955, a foamed starch product is produced by extruding mixture of starch and gas releasing agent. The foamed starch product may be used as foam sheets or cushioning fillers for packing.
In Patent Cooperation Treaty (PCT) Publication No. WO 97/10293, a process is disclosed for the production of shaped bodies, in particular packaging shaped bodies, from biologically decomposable material using a viscous mass containing biologically decomposable fiber material, particularly waste paper fiber, water and starch. This viscous mass is baked in a closed mould to produce a shaped body. This shaped body is then subjected to liquid-impermeable coating to increase water resistance of the shaped body.
While the disclosures mentioned above show the use of gas releasing agent to create a cell structure of the product and the use of waste paper fiber to strengthen the shaped bodies, the present invention discloses the method of preparation foam-liked products from the non-synthetic biodegradable starch-based composition without using gas releasing agent. Waste paper pulp may not be economically feasible in some countries for this kind of product, the present invention relates to the application of limestone powder and cassava pulp to make the product stronger and more dimensionally stable at a lower cost.
The non-synthetic, biodegradable starch-based composition described in the present invention is a much better improvement over the invention of Wang (U.S. Pat. No. 5,922,379) in that Wang has described the protein/starch thermoplastic composition whose main component is protein either of plant or animal origin 10–46 or 50 wt % which is quite high and makes the product very costly, renders the product not commercially feasible and much more difficult to process or handle. Such high protein compositions of the product as described by Wang can never be used under the forming condition using quite high temperature of 150–300 degree Celsius and the compression technique described in the present invention due to the fact that optionally incorporation of any protein greater than 5 wt % causes the shaped bodies to become brittle, charred and burnt before the starch in the mixer becomes fully gelatinized, thus causes the failure of manufacturing process. This is also the reason that some starch or flour need to be especially chosen to limit the protein content of not greater than 5 wt %. In addition, in some combination of the ingredients, protein may not be necessary at all as indicated by 0% by weight of protein added to minimize the cost yet giving the product most desirable. Thus, protein is not the main component of the shaped bodies presently described which is significantly different from that of Wang's. Adding high protein of any origin in the compositions of Wang's makes it not possible to manufacture under the condition described in the present invention with too many serious disadvantages.
Firstly, it makes the product become too costly and not possible to commercialize as the cost of the product is not competitively feasible compared to those non-biodegradable foam plastic containers presently available in the market. Secondly, under the presently described condition where mixing and kneading until uniform mixture is obtained before being injected or compressed into a closed mould of temperature ranging 150–300 degree Celcius to allow full gelatinization of starch to be accomplished, such high forming temperature used in the present invention causes the denaturation of any high protein composition greater than 5% by weight and destroys the shape bodies. Thirdly, the charred shaped bodies got stuck to the mould surface and causes all the troubles which makes the manufacturing process not possible. Fourthly, if these high protein compositon is really needed, to reduce the troubles to a certain degree but not all is to add quite a high quantity of lubricant which in turn would additionally increase the cost of production significantly. All these crucial reasons make manufacturing the protein/starch-based thermoplastic composition described by Wang not possible, in addition that it is not commercially feasible.
The non-synthetic biopolymers used as the main ingredients in the present invention with no addition of any of the synthetic plastic polymers like EAA or acrylic acid derivatives or polyethylene makes the present invention distinctly different than that of Schiltz (U.S. Pat. No. 5,449,708). This is a great improvement which leads to a big impact regarding the residue left in the environment. The shaped-bodies of the present invention after being used as containers for food or any other materials, can be used further for cattle and pig feeding or may be used as fertilizer leaving nothing at all and no harm to the environment. Contrast is true for Schiltz's invention as their products although biodegradable still leave a lot of synthetic polymers (49.8% of the mass) very harmful to the environment as they accumulated. In other word, the product described in the present invention is 100% biodegradable while Schiltz's is not.
Therefore, the present invention describes a non-synthetic, biodegradable starch-based composition and a method for production of shaped bodies which is a great improvement over the prior arts where injection or compression into a closed mould instead of the extrusion process with a foam die or baking used in prior arts.